Conserving Bats and Buildings

A Natural Synergy

Kelly Gunnell

  Flash-lit photograph of bat in flight  
  A pipistrelle bat in flight (Photo: Hugh Clark)  

Long associated in popular culture with horror and the supernatural, bats are in desperate need of a PR makeover. Contrary to popular opinion, bats are not mice with wings. They are, in fact, more closely related to humans than to rodents. These socially complex mammals usually have just one young per year and look after their offspring fastidiously, often forming crèches that allow the mothers to go out to forage while other females look after the pups.

Bats have long life-spans for their size. The record is 40 years for a ringed Brandt’s bat. They do not make nests and they do not cause structural damage to buildings. Bats are not blind, they can see reasonably well but have evolved echolocation to help them with the difficult task of hunting tiny insects in the dark. Bats are an important part of the UK’s biodiversity, the 17 species of bat account for nearly one third of our national mammal species (globally there are over 1,000 bat species). All bats in the UK eat insects – a single pipistrelle bat can eat up to 3,000 midges in a single night. Bats are thus an important part of the ecosystem and their presence is an indication of a healthy and biodiverse landscape.

One stereotype does hold true, however: bats do like to roost in old buildings. In fact, bats and people have been sharing dwellings for thousands of years, making these unique creatures part of our cultural heritage as well as part of our natural heritage. Consequently, when conserving old buildings, there is a legal and moral imperative to preserve bat roosts.


  Barn exterior  
  (Photo: Pat Waring/Ecology Services UK Ltd)  
  A large aisled barn in East Lancashire, dating from 1605, was undergoing restoration. The roof structure was in a poor condition and needed substantial repairs and refurbishment. The building comprised coursed sandstone rubble with quoins and a stone-slate roof. Bat roosting features included wall cavities and a very large number of mortice joints in the rafters, aisle posts and kingpost roof supports. Bat species found in the barn included common and soprano pipistrelle, brown long-eared and Natterer’s Bats. As part of the preparations for roof repairs, surveys were undertaken throughout the year in 2009, using a combination of building searches, remote detectors (every month) and manned surveys (May to September). Bats were found to be active in all months of the year and the building supported roosting bats in every month. The information gained will be an essential part of the mitigation strategy.

About half of the bat species in the world use holes in trees for roosting. The other half use either caves or cavities. However, as ‘natural’ roosting sites have become scarce due to development and changing land-use, bats have adapted to use buildings, like barn owls or swifts, and are a building-reliant species.

Buildings offer a range of possible roosting opportunities that mimic those found in the natural environment and are often cleaner, safer and warmer spaces than natural roosting sites. Crevice-like or tree cavity type spaces include those found in both modern and traditional houses such as behind fascia and barge boarding, spaces beneath roof tiles, wall coatings, hollow mortice joints, rain gutters and chimneys. Cave-like spaces include attics and cellars that are dark, with stable temperatures and humidity. These spaces may be found in a range of structures including farm buildings, historic houses, castles, churches and terraced houses.

Bats are much smaller than people tend to think; some species only need a 15-20mm gap to access a roosting space. Most bat species cluster together tightly to generate heat and, given that it’s possible to squeeze about 200 pipistrelle bats into a space the size of a shoe box, you can imagine how difficult it can be to find bats roosting in the dark corners of a building.

Bats do not make nests or chew electrical cables. The most obvious sign of their presence is bat droppings but even these can be hard to find, especially in a loft. Bat droppings are frequently mistaken for mouse droppings but can be identified by the ‘crumble test’. Bat droppings consist largely of insect remains and crumble easily to a powder of semi-shiny fragments. Rodent droppings are smooth and plastic, quickly becoming hard. They cannot be crumbled. The droppings of UK bats do not present any known health hazards; they can be swept up safely and make an excellent garden fertiliser. Large accumulations may reflect use of the same roost over a number of years rather than large numbers of bats at any one time.

Other signs to look for are grease marks on the rafters, urine splashes, cobweb-free corners, or insect remains from a feeding perch. If in doubt, contact your local bat group or a professional ecologist to conduct a survey.

Unfortunately for bats, even man-made roosts are now under threat. Demolition of old buildings, renovation, change of use, artificial lighting and the move towards airtight buildings, all have implications for the bat populations that use buildings. Combined with the loss of foraging habitat, this has meant that bat populations have suffered drastic losses in the last century. Bats are therefore afforded very strict legal protection which must be considered when carrying out any building work.


  Mill exterior with woodland in backfground and road in foreground
  (Photo: Pat Waring/Ecology Services UK Ltd)  
  A former textile mill, built in 1888 in a river valley in West Yorkshire, was proposed for development into a series of apartments. The surrounding landscape is ideal for bats and includes at least six species. Within the mill buildings, common and soprano pipistrelle bats have been recorded during spring and summer months, with at least one of the species hibernating in ground floor door lintels. The buildings also supported nesting swifts. As part of the preparation for development a dedicated bat barn was constructed in 2008 within 100m of the mill, adjacent to woodland and the river. Subsequent monitoring found bat droppings in the bat barn in 2010.

European and domestic legislation dictates that any structures or places which bats use for shelter are protected from damage or destruction, whether occupied by bats or not. This legislation means that planning authorities have an obligation to consider whether bats are likely to be affected by a proposed development. An application cannot be granted planning permission unless it includes all the information needed to make sure that bats will not be affected by building works.

It is for this reason that planning authorities will often ask for a bat survey to be carried out and the results submitted with the initial planning application. The survey will need to be undertaken by a licensed ecologist, who will inspect the site and building internally and externally to determine the likelihood of bats being present.

If the potential for the site to support bats is considered high, additional activity surveys will be carried out at dawn or dusk. These surveys aim to confirm the species present, the type of roost and identify any access points and commuting routes. Once this information has been collected, an assessment is made as to whether the planned works will impact the bats. If necessary, measures will then be suggested to reduce the impact to an acceptable level. In some instances where the impact is considered significant, an application for a European Protected Species (EPS) licence will be needed. The EPS licence will outline conditions to mitigate for the unavoidable impacts. This does not stop the work from going ahead, but tries to ensure that any work doesn’t negatively impact the bats and is within the law.

The licensing process can take up to six weeks so it is always recommended that an ecologist is contacted as early on in the process as possible. This allows for any alterations to the scope and timing of works to be incorporated into schedules and prevents delays.


When undertaking building or development work where bats may be present, the following steps should be taken:

  • contract an ecological consultant for guidance throughout the project
  • undertake a bat survey at the appropriate time of year
  • if bats are present, compile a mitigation/ compensation plan or method statement for everyone involved including architects and building contractors
  • incorporate the bat survey report and mitigation plan/method statement into the planning application
  • apply for planning permission
  • if necessary, apply for an EPS licence (the planning permission will be needed as part of the licence application)
  • if a licence is granted, carry out works with an ecologist’s supervision
  • carry out a compliance check to ensure that mitigation is being properly implemented
  • monitor the site after the completion of the mitigation to gauge the response of the bat population.


Ashlar lodge with polygonal turrets and embattled parapet (c1840)
(Photo: CTT Sustainable Architect)  
Clarkencombe Lodge is a Grade II* listed gatehouse and part of the Ashton Court Estate in Bristol. Plans to refurbish the lodge as a residential space had to allow for the five species of bat found on the site to continue to use the building and to enhance the maternity roost. The bat species included greater and lesser horseshoe bats, brown long-eared, common pipistrelle and barbastelle bats. Mitigation involved isolating the living accommodation from the designated bat areas, including the roof void. One tower of the gatehouse was also allocated for the bats, creating a route from the roof to the ground level single storey modern extension, which was modified to provide roosting ledges and a maternity environment. All windows and doors were removed from the extension and the building covered with planting. Its interior layout was designed by a bat consultant. Bat entrances and exits were created in the roof and tower. Canopy cover from the building to the adjacent woodland was ensured. A camera installed for ongoing monitoring has shown an increase in the use of the building by bats and that the improved maternity roost is in use.

The main aim of bat roost mitigation is to allow for the conservation status of bats to be maintained or enhanced during and after the development.

Bat roost mitigation comes in many forms. In most cases, mitigating for bats during building works involves changing the construction schedule to avoid sensitive breeding seasons and changing the scope of the work to maintain the bat entrances. Bat access points are created or maintained by making gaps either using existing materials or specially designed bat access tiles or bricks that allow bats to re-enter the roost once the work is finished.

More complicated forms of mitigation involve replacing roosts within the building or creating purpose-built roosts such as bat houses or bat barns. Maintaining roosts in situ is always the preferred option. However, in some cases purpose-built bat houses may be considered, as long as the risks of non-adoption by bats are minimised through careful design and site selection. In all cases, bat mitigation should aim to mimic or optimise the original roosting conditions, particularly thermal properties and access to favourable landscape features.

Below are some design principles for ex situ roost conservation:

  • situate the replacement roost as close as possible to the roost to be lost
  • match the replacement roost closely in terms of size, height and aspect
  • situate the replacement roost close to existing flight-lines and have an entrance close to appropriate habitat to maximise chances of the bats finding and adopting it
  • design the new roost to provide a suitable thermal regime for the target bat species
  • provide a variety of roosting opportunities and thermal regimes
  • make the building resistant to vandalism
  • arrange for the long-term integrity and security of the replacement roost.


The frustration for homeowners and ecologists alike is that even the best thought-out and designed roost mitigation may not succeed.

Bats can be fickle and our understanding of them is incomplete. Each species has its own unique preferences for temperature, roost size, access points and proximity to suitable landscape features and vegetation. These requirements can also change regionally and seasonally. Determining all these criteria and putting them together for a successful mitigation strategy is immensely challenging.

Combine this with a lack of systematic follow-up and monitoring and you can see why the realm of bat roost mitigation has remained more of an art than a science. The licensing procedure has made it difficult for builders, ecological consultants, architects and homeowners to keep track of the mitigation measures used in the past and whether they were successful.

  Bat access point in stone turret: the access point is a horizontal rectangular opening edged with lead flashings  
  New bat access in one of the turrets of Clarkencombe Lodge, Bristol
(Photo: CTT Sustainable Architect)

To address this, the Bat Conservation Trust has developed a new web resource called Roost which enables users to upload information about roosts, mitigation strategies and explore case studies. Users can browse case studies using a number of criteria, make comments and access information on general design principles and resources for bat roost mitigation. The aim is that Roost will become a knowledge hub for anyone undertaking or involved in bat roost mitigation. In the long term, the collected information could provide a wealth of insight into what it takes to make a successful and attractive replacement roost.

It is hoped that by sharing information about effective mitigation, we can find new ways to enable bats to live happily alongside us for centuries to come.

Roost was launched in September 2011 (



Recommended Reading

  • Bats and Buildings: Bats and the Built Environment Series Volume 1, Bat Conservation Trust, 2010
  • Bat Mitigation Guidelines, Natural England, 2004
  • Bats in Traditional Buildings, English Heritage, National Trust and Natural England, 2009
  • C Williams, Biodiversity for Low and Zero Carbon Buildings: A Technical Guide for New Build, RIBA Publishing, London, 2010



The Building Conservation Directory, 2012


KELLY GUNNELL is the built environment officer for the Bat Conservation Trust. The
Built Environment Project aims to raise awareness and facilitate solutions for bat conservation in the construction and building sector.

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